1. Field of the Invention
The present invention relates to the photolithography process used in the manufacturing of semiconductor devices. More particularly, the present invention relates to a method of and an apparatus for exposing a semiconductor wafer to transcribe a pattern onto the wafer.
2. Description of the Related Art
Photolithography is one of the major processes of manufacturing a semiconductor device as it is indispensable to the overall process of forming a particular circuit on a wafer. In general, photolithography includes a series of individual processes such as a process of coating a substrate with a photoresist, an exposure process in which the photoresist is exposed to light of a given wavelength, and a process of developing the exposed photoresist. In the exposure process, light is directed through a mask having a particular pattern to transcribe the pattern onto the wafer.
The higher the degree of integration desired for a semiconductor device, the finer are the patterns that must be produced on the wafer. Thus, photolithography is becoming increasingly critical in the overall process of manufacturing semiconductor devices. In particular, the demand for providing more highly integrated semiconductor devices has triggered the need to develop an exposure apparatus and method that can provide a pattern having high degree of resolution and optimum depth of focus (DOF). In general, off axis illumination (OAI) is used in photolithography to secure a high degree of resolution and optimal DOF.
FIG. 1 illustrates a conventional projection-type of exposure apparatus. The projection-type exposure apparatus includes a light source unit 110, an optical lens unit 120, an aperture 130, and a pattern transfer unit 140. The pattern transfer unit 140 includes a plurality of lenses and thus may be regarded as being a part of the optical lens unit 120. However, in this disclosure, the pattern transfer unit 140 will be referred to separately to be differentiate it from the optical lens unit 120, in consideration of its dedicated function of transferring the pattern of the photomask 144 thereof onto a wafer.
The light source unit 110 includes a light source 112 and oval mirrors 114 that encompass the light source 112. Light having a particular wavelength is emitted from the light source 112. The light emitted from the light source 112 in various directions is reflected in one direction by the oval mirrors 114.
The optical lens unit 120 includes a collecting lens 122 and a fly's eye lens 124. The collecting lens 122 focuses light emitted from the light source 112 into parallel light rays, and the fly's eye lens 124 focuses the parallel rays of light such that they will be uniformly incident on a target object.
The light passes through the fly's eye lens 124 and travels toward the aperture 130 before the light reaches a condensing lens 142 of the pattern transfer unit 140. The aperture 130 has two regions: an open area through which the light passes and a blocking area that blocks the light. The open area of the aperture 130 has a specific shape. In OAI, a vertical component, i.e., the 0th order, of incident light is removed using the specific shape of the aperture 130. Therefore, light passing through the aperture 130 is incident on the photomask 144 via the condensing lens 142 at a predetermined oblique angle rather than at a right angle.
FIG. 2 illustrates various types apertures used in a conventional projection-type of exposure apparatus. In FIG. 2, the cross-hatched portions denote the blocking regions of the apertures. The conventional illustrated apertures are a circular aperture, a quadrupole type of aperture, a dipole type of aperture, and an annual aperture. However, other types of apertures are also known.
Referring again to FIG. 1, the light passing through the aperture 130 is condensed by the condensing lens 142. The condensed light rays are incident on the photomask 144 that bears a mask pattern. Next, the condensed rays of light passing through the photomask 144 pass through a projecting lens 146 and are finally focused on a semiconductor wafer 150 disposed on a wafer holder 160.
However, the aperture 130 must be tailored to the pattern formed on the photomask 144 in order to obtain a pattern having the highest resolution and optimum DOF using OAI. That is, if the photomask 144 is changed to one whose pattern has a different size, shape, and/or spacing, the aperture 130 must be replaced with one that is matched to the new pattern.
In general, 20–30 sheets of photomasks are used to manufacture one integrated semiconductor device, whereas a projection-type of exposure apparatus is equipped with only several apertures. If necessary, an aperture 130 may be detached from the projection-type of exposure apparatus and replaced with a new aperture.
Accordingly, a conventional exposure apparatus and method have some disadvantages. First, the shapes of the available apertures are limited. That is, only several types of apertures are available and thus, there is a high probability that none of the available apertures is an optimal match for the pattern of the selected photomask. Accordingly, in most cases, it is difficult to precisely transcribe the pattern of a photomask and form a pattern having the highest resolution and optimum DOF on a wafer using a conventional exposure apparatus and method.
Secondly, the conventional exposure apparatus is inconvenient in that the aperture must often be exchanged during the process of manufacturing a semiconductor device. The operation of the exposure apparatus must be temporarily discontinued while the apertures are being exchanged, thereby increasing the total time of the exposure process and consequently lowering the productivity of the manufacturing process.
Thirdly, as described above, an aperture has two regions: an open region that allows light to pass through the aperture, and a blocking region that prevents light from penetrating the aperture. In other words, not all the light that is incident on the aperture passes through the aperture to the semiconductor wafer. Accordingly, the conventional exposure process is not marked by energy efficiency meaning that the exposure time must be long to satisfactorily complete an exposure process. Therefore, the overall manufacturing process using the conventional exposure method and apparatus also takes a long time to complete.